1. Field of the Invention
The present invention relates to a viewfinder optical system for a camera.
2. Description of the Prior Art
In general, single-lens-reflex cameras utilize relay-type viewfinder optical systems. Conventional relay-type viewfinder optical systems are composed of a number of lenses, and this has prevented the reduction of the weight, size, and cost of such viewfinder optical systems. These inconveniences can be overcome by reducing the number of constituent lens elements.
However, the reduction of the number of lens elements and the use of plastic lenses make it more difficult to correct aberration satisfactorily. In particular, chromatic aberration, which can be corrected only with a combination of lenses made of materials having different dispersion, is difficult to correct with a reduced number of lens elements or with plastic lenses. In addition, since there are only few kinds of plastic that can be used satisfactorily as lenses, it is not possible to freely select plastic materials having different dispersions for the correction of chromatic aberration. Accordingly, it is difficult to use plastic lenses for all the lens elements constituting an optical system.
On the other hand, lens-shutter cameras and digital cameras employ Kepler-type real-image viewfinder optical systems. Some conventionally known Kepler-type real-image viewfinder optical systems include an objective lens and an eyepiece lens, and correct chromatic aberration by the use of a diffracting optical surface. For example, U.S. Pat. No. 5,044,706 proposes a viewfinder optical system in which a binary diffraction grating is provided only in the objective lens, and U.S. Pat. No. 5,446,588 proposes a viewfinder optical system in which a diffracting optical element is provided only in the eyepiece lens.
The optical performance of a Kepler-type real-image viewfinder optical system is evaluated by evaluating its total optical performance including that of the objective lens and that of the eyepiece lens. Accordingly, for example, the axial chromatic aberration occurring in the objective lens is added together with that occurring in the eyepiece lens for evaluation. In addition, when the objective lens is a zoom lens system, the objective lens needs to be designed such that chromatic aberration is properly corrected over the entire zoom range.
In the viewfinder optical system proposed in U.S. Pat. No. 5,044,706, since a diffracting optical surface is provided only in the objective lens, the chromatic aberration occurring in the eyepiece lens needs to be corrected by the use of negative lenses in the eyepiece lens. Accordingly, this viewfinder optical system inevitably requires more lens elements because of the extra negative lens elements used in combination.
On the other hand, in the viewfinder optical system proposed in U.S. Pat. No. 5,446,588, a diffracting optical surface is provided only in the eyepiece lens, and the optical performance of the optical system is evaluated by evaluating only the optical performance of the eyepiece lens. Accordingly, to achieve proper correction of chromatic aberration over the entire system, this viewfinder optical system needs to be provided with additional negative lenses for correcting the chromatic aberration occurring in the objective lens. This inevitably increases the number of lens elements.